heap: When verifying & swapping a fill pattern, stop swapping if verification fails
heap: When verifying & swapping a fill pattern, stop swapping if verification fails

Avoids situation where data has been overwritten during any post-mortem analysis
(core dump, gdbstub, etc).

As reported via forum https://esp32.com/viewtopic.php?f=2&t=6471&p=27790

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